This paper deals with concepts and methods concerning the use of the Impact Factor (IF), as defined by the Institute for Scientific Information (ISI), on the evaluation of scientific production published in specialized journals. The history of IF is discussed from its first formulation up to the moment when it became the subject of a large amount of investigations regarding the differences among the values for different knowledge areas. It is emphasized that the main variables affecting IF are journal density and their obsolescence rate. The synchronous and diachronous approaches used to measure obsolescence are discussed, such as the immediacy index, half-life of citations and citation age. As a conclusion, the Brazilian system for scientific evaluation is analyzed, as well as the SciELO role on the formulation of bibliometric indicators.